Ankylosing spondylitis

Wikipedia has articles on both spondylitis and ankylosing spondylitis, which is basically an autoimmune arthritis of the spine that tends to have its first onset in young adults. It is considered to be highly heritable, with over 90% of the risk of developing the disease being genetically based. It is two to three times more prevalent in men than in women, and men tend to be more severely affected.